What is the ruling on praying on pieces of iron or on a bridge over water near dry land?

The first (option) for someone who can pray on dry land away from the bridge is to do so, in order to achieve humility (khushu'). However, praying on the bridge is valid if all its conditions and pillars are fulfilled. The Prophet (peace be upon him) prayed on a mat and prostrated on a small prayer mat (khumrah).